Description Nearly all new human immunodeficiency virus (HIV) infections are acquired by sexual transmission. Immediately after infection, the virus population of the recipient is at its smallest, most homogenous, and probably most susceptible most susceptible stage to immunological clearance. Thus, the development of vaccine strategies designed to elicit protective immunity at mucosal surfaces has become a high priority within the AIDS research community. Yet, fundamental questions regarding the mechanisms of mucosal transmission relevant to vaccine development remain unanswered. The first cells that become infected during mucosal HIV transmission have not yet been identified, and it is not clear whether injection by mucosal routes results in the selective transmission of certain types of viruses of certain types of viruses. We propose to address these questions experimentally by mucosal inoculation of rhesus macaques with simian immunodeficiency virus (SIV) recombinants that express enhanced green fluorescent protein (EGFP) or enhanced blue fluorescent protein (EBFP), and are only capable of a single cycle of injection and gene expression. In the first specific aim, we will identify the first cells infected in the vaginal and rectal mucosa by co- localization of EGFP expression with fluorescent antibody staining for cell type-specific antigens. In the second specific aim, we will test the hypothesis that macrophage tropic (M-tropic) viruses are transmitted more efficiently across mucosal barriers than T lymphocyte tropic (T- tropic) viruses by inoculating macaques with equal mixtures of and T- tropic single-cycle chimeric simian-human immunodeficiency (SHIV) recombinants that express either EGFP or EBFP. By comparing the frequency of EGFP to EBFP expressing cells present in the vaginal and rectal mucosa, we will be able to determine whether or not M-tropic viruses are selectively transmitted during mucosal infection.